e997025.pdf

pull-up accelerator

025

LTC1694CS5

IC1

100n

LTC1694

BUS1

BUS2

SCL

SOT-23

SDA

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K.S.M. Walraven

Systems like the SMBus or I 2 C™ use a standard resistor to pull up

the signal levels to the positive supply rail (normally 5 V). The bus

goes low because an appliance connected to it pulls the signal to

zero via its open-collector output. The well-known problem is this

output can draw a much higher current than the pull-up resistor

can compensate. This results in a steep trailing edge, but a much

more gradually rising leading edge, whose transition in addition is

not linear but exponential. This adversely affects the duty factor

of the signals and also reduces the speed of the bus.

Linear Technology have available an IC (Type LTC1694) to

replace the traditional pull-up resistor which can produce a cur-

rent that is dependent on the changes in level taking place on the

bus. When that level rises, the IC gives 2.2 mA, but when it falls,

the current is only 275 µA.

Since the IC contains two circuits for replacing both pull-up

resistors, it is possible to detect when the bus is in the quiescent

mode (both pull-ups high). In this case, the current is reduced even

more: to 100 µA.

The IC is intended for an I 2 C bus frequency of 100 kHz. The

400 kHz and recently introduced 3.4 MHz versions are not sup-

ported. The IC is housed in a SOT-23 case.

The characteristic shows the difference of the leading edge

obtained with a standard resistor and that resulting when the IC

is used.

[994076]

1V/div.

V CC = 5V

C LD = 200pF

f bus = 100kHz

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